The present invention relates to a lightning arrester device for power transmission line which is disposed on a steel tower for protection of an AC aerial power transmission cable.
In usual, a lightning arrester is disposed on a steel tower in order to protect an aerial power transmission cable from lightning. It is preferable to use a compact size of a lightning arrester, because of space problem.
A non-dynamic current type lightning arrester can be formed by using a sintered element having excellent non-linear resistive characteristic which is made of a main component of zinc oxide as a lightning arrester element, and a serial gapless compact lightning arrester can be obtained as disclosed in U.S. Pat. No. 3,806,765.
It has been considered that a zinc oxide type lightning arrester is optimum for protection of a power transmission line.
On the other hand, a shielding from lightning has been attained by using an aerial ground wire laid at a top of the steel tower in an aerial power transmission line system.
When the aerial ground wire is struck by lightning, a potential at the steel tower is instantaneously raised whereby a reverse flashover is applied to the power transmission cable in a case of no lightning arrester. When a lightning arrester is connected, the voltage applied to the supporting insulator for supporting the power transmission cable can be controlled to prevent the reverse flashover.
A lightning current passing through a lightning arrester is about 5 KA when a lightning having a peak value of 100 KA is struck to an aerial ground wire near the steel tower in two circuits of 275 KV.
In the case of the zinc oxide type lightning arrester, the lightning arrester is non-dynamic current type whereby it is enough to treat only the impulse current of about 5 KA and the duty for the operation is lower than the duty for the spark current of 10 KA as a lightning arrester for a substation. Therefore, if the lightning can be completely shielded by the aerial ground wire, only the duty of the lightning arrester is required. However, in practice, a failure of shielding is caused. For example, when the power transmission line is struck by the lightning of 100 KA near the steel tower, a lightning current of about 90 KA is passed through the lightning arrester in the phase of the cable on the steel tower. According to statistic data, in about 5% of lightnings, the lightning current of more than 100 KA is given.
In the conventional lightning arrester, about 90% of the direct lightning current should be arrested by the lightning arrester, whereby the operation duty is too heavy and sometimes, the lightning arrester is disadvantageously damaged.